Some conventional vehicles include all-wheel-drive capabilities. These vehicles power two wheels during high-traction situations to enhance fuel-economy, and power all four wheels during reduced-traction situations to enhance road traction and stability. Torque-biasing devices are conventionally used to transfer the torque output from the engine source away from a first wheel and towards a second wheel during the reduced-traction situation.
During a reduced-traction situation, one of the wheels of the vehicle often has a much faster rotational speed than another wheel. Torque-biasing devices are conventionally controlled based upon the difference between the rotational speeds of a first wheel and a second wheel. More specifically, the torque-biasing devices are conventionally controlled based upon a mathematical equation including a proportional term and an integral term of this rotational speed difference.
The proportional term of the control allows the torque-biasing device to understand the current state of the vehicle. Quite simply, the torque-biasing device is activated if there is a large difference between the rotational speeds of the first wheel and the second wheel. Once the torque-biasing device is activated, the vehicle powers all four wheels and the rotational speed difference may approach zero. When the rotational speed difference approaches zero, the proportional term approaches zero and—without the integral term—the torque-biasing device is de-activated. The vehicle, however, may still be experiencing a reduce-traction situation. For this reason, the torque-biasing device needs to “remember” the past situations. This is conventionally accomplished with the integral term.
The integral term allows the torque-biasing device to understand the past states of the vehicle. Since both the proportional term and the integral term conventionally use an absolute value of the rotational speed difference, the integral term is a positive value, which will always effect the control of the torque-biasing device despite the time elapse between reduced-traction situation and the current state of the vehicle. Thus, there is a need in the field of torque-biasing systems to provide a torque-biasing system that balances the need to “remember” the past situations, but eventually “forgets” them to avoid any negative affect on the torque-biasing device.